1. Field of the Invention
The present invention relates to a switching power supply device for supplying a stabilized DC voltage to industrial and consumer electronic apparatuses.
2. Description of the Related Arts
The recent tendency toward electronic apparatuses of lower cost, smaller size, higher performance and higher energy efficiency has resulted in a strong demand for switching power supply devices which are more compact, stable in their output and highly efficient.
A conventional switching power supply device will now be described. FIG. 10 shows a circuit configuration of a half bridge converter which is a conventional switching power supply device. In FIG. 10, 1 represents an input DC power supply whose voltage is represented by V.sub.IN. 2 and 2' represent input terminals to which the input DC power supply 1 is connected. 3 represents a first switching element, and 5 represents a second switching element. The first switching element 3 and second switching element 5 are alternately and repeatedly turned on and off and are connected to the input terminals 2 and 2' in series. 18 and 19 represent first and second capacitors, respectively, which are connected to the input terminals 2 and 2' in series. The electric potential of the connection point between the first capacitor 18 and second capacitor 19 is represented by V.sub.C. 20 represents a transformer which has a primary winding 20a, a first secondary winding 20b and a second secondary winding 20c. The same turn ratio n:l is employed between the primary winding 20a and the first secondary winding 20b and between the primary winding 20a and the second secondary winding 20c. The primary winding 20a of the transformer is connected to the connection point between the first switching element 3 and the second switching element 5 at one end thereof and is connected to the connection point between the first capacitor 18 and the second capacitor 19 at the other end thereof. 21 and 22 represent first and second rectifier diodes, respectively, whose anodes are connected respectively to the first secondary winding 20b and the second secondary winding 20c of the transformer and whose cathodes are connected to each other. 11 represents an inductance element, and 12 represents a smoothing capacitor. The inductance element 11 and the smoothing capacitor 12 are connected in series, and the resultant series circuit is connected to the connection point between the first rectifier diode 21 and the second rectifier diode 22 at one end thereof and to the connection point between the first secondary winding 20b and the second secondary winding 20c of the transformer 20 at the other end thereof. This series circuit smoothes a voltage which has been rectified by the first rectifier diode 21 and the second rectifier diode 22 to provide an output voltage. 13 and 13' are output terminals. The electrostatic capacity of the smoothing capacitor 12 is sufficiently large, and an output voltage V.sub.O is outputted to the output terminals 13 and 13'. 14 represents a load which is connected to the output terminals 13 and 13' and consumes the electric power. 15 represents a control circuit which drives the first switching element 3 and the second switching element 5 at predetermined on-off ratios to stabilize the output DC voltage V.sub.O.
The operation of a switching power supply device having the above-described configuration will now be described (see FIGS. 11(a), 11(b), 11(c), 11(d), 11(e), 11(f), and 11(g). When the first switching element 3 is on, the voltage V.sub.C is applied to the primary winding 20a of the transformer to produce a voltage V.sub.C /n at the first secondary winding 20b of the transformer. As a result, the first rectifier diode 21 is turned on; the second rectifier diode 22 is turned off; and a voltage V.sub.C /n-V.sub.O is applied to the inductance element 11. A current flows through the first switching element 3, which is the sum of the primary-converted values of the excitation current of the transformer 20 and the excitation current of the inductance element 11. When the first switching element 3 is turned off, the secondary current is divided into separate flows through the first secondary winding 20b and the second secondary winding 20c so that the excitation energy of the transformer 20 will become continuous; the first rectifier diode 21 and the second rectifier diode 22 are turned on; induced voltages of the first secondary winding 20b and the second secondary winding 20c become zero; and a voltage -V.sub.O is applied to the inductance element 11. Then, as the second switching element 5 is turned on, a voltage V.sub.IN -V.sub.C is applied to the primary winding 20a of the transformer; a voltage (V.sub.IN -V.sub.C)/n is generated at the second secondary winding 20c of the transformer; the first rectifier diode 21 is turned off; the second rectifier diode 22 is turned on; and a voltage (V.sub.IN -V.sub.C)/n-V.sub.C is applied to the inductance element 11. A current flows through the second switching element 5, which is the sum of the primary-converted values of the excitation current of the transformer 20 and the excitation current of the inductance element 11. When the second switching element 5 is turned off, the secondary current is divided into separate flows through the first secondary winding 20b and the second secondary winding 20c so that the excitation energy of the transformer 20 will become continuous; the first rectifier diode 21 and the second rectifier diode 22 are turned on; induced voltages of the first secondary winding 20b and the second secondary winding 20c become zero; and a voltage V.sub.O is applied to the inductance element 11 in the opposite direction. If the on-off ratios of the first switching element 3 and the second switching element 5 are set so that they have the same on time T.sub.ON and if both an off time from the turn off of the first switching element 3 to the turn on of the second switching element 5 and further an off time from the turn off of the second switching element 5 to the turn on of the first switching element 5 are the same on time T.sub.OFF as shown in FIG. 11, the magnetic flux of the transformer 20 is reset after each cycle in a steady operational state. As a result, the following equation is obtained. EQU (V.sub.IN -V.sub.C).times.T.sub.ON =V.sub.C .times.T.sub.ON
Therefore: EQU V.sub.C =V.sub.IN /2
The following equation is derived from the resetting conditions for the inductance element 11. EQU (V.sub.IN /2-V.sub.O).times.T.sub.ON =V.sub.O .times.T.sub.OFF
Therefore: EQU V.sub.O =.delta.V.sub.IN /2
where .delta.=T.sub.ON /(T.sub.ON +T.sub.OFF). That is, the output voltage V.sub.O can be stabilized by adjusting the on-off ratios of the first switching element 3 and the second switching element 5. Various operational waveforms of FIG. 10 components are shown in FIG. 11.
This circuit configuration is characterized in that any voltage higher than the input voltage is not applied to the first switching element 3 and the second switching element 5 and in that the transformer 20 is not DC-excited.
However, the conventional configuration described above has a problem in that a power loss is caused because a surge current is generated by parasitic capacities of the switching elements and a distributed capacity of the transformer which are shorted when the first switching element 3 and the second switching element 5 are turned on.